In the present paper we show that the Hamiltonian describing the resonant interaction of N two-level systems with a single-mode electromagnetic quantum field in the Coulomb gauge can be diagonalized with a high degree of accuracy using a simple basis set of states. This allows one to find an analytical approximation for the eigenvectors and eigenvalues of the system, which interpolates the numerical solution in a broad range of the coupling constant values. In addition, the introduced basis states provide a regular way of calculating the corrections and estimating the convergence to the exact numerical solution. The obtained results are valid for both quantum Rabi model (N = 1) and the Dicke model for N ≥ 2 atoms.

We compute a spectrum of parametric X-ray radiation (PXR) inside a crystal from a bunch of electrons, which is periodically modulated in density. We consider that the bunch of electrons is exiting from a XFEL channel. We demonstrate that in the case of a resonance between the frequency of parametric X-ray radiation and a frequency of modulation of an electron bunch the sequence of strong quasi-monochromatic X-ray pulses is formed -superradiant parametric X-ray emission (SPXE) with frequencies multiples of the modulation frequency. The number of photons in the impulse of SPXE in the case of an extremely asymmetric diffraction is comparable with the photon number in the impulse of a XFEL. Moreover the SPXE is directed under the large angle to the electron velocity and every harmonic in the spectrum is emitted under its own angle.

The solution for the large-radius Fröhlich polaron in the Schrödinger representation of the quantum theory is constructed in the entire range of variation of the coupling constant. The energy and the effective mass of the polaron are calculated by simple algebraic transformations and are analogous to the results found by Feynman on the basis of the variational principle for the path-integrals of this system. It allows us to solve the long-lived problem of the inequalities of the functional and operator approaches for the polaron problem. The developed method is important for other models of particle-field interaction including those ones for which the standard perturbation theory is divergent.